As inorganic hollow powder, there is spherical hollow glass powder. The spherical hollow glass powder has, for instance, a low specific gravity, low dielectric characteristics, high heat resistance and excellent thermal insulation properties, as compared with that of the inorganic non-hollow or solid powder. The applications of such inorganic powder, which make the most use of their light weight properties, include, for instance, molded resin parts such as those used in automobiles and portable electronic apparatuses; construction parts such as sealing compounds and exterior wall materials; and structural materials such as buoyancy materials for ships, while the applications thereof, which make the most use of the low dielectric characteristics of the powder include, for instance, multi-layer print-circuit boards, cladding materials for electric wires, and sealing agents for semiconductor devices. The recent requirements for the hollow glass powder are to further micronize the powder, to increase the rate of hollow particles (the hollowness rate of particles) and to realize or provide inorganic hollow powder made of inorganic materials other than the vitreous materials.
As a method for the preparation of hollow glass powder, there has conventionally been proposed one which comprises the step of firing silica gel powder on which a glass-forming agent and a foaming agent have previously been supported (see, Patent Document 1). This method makes it possible to form powder which has a particle density of about 0.3 g/cm3 and an average particle size of about 70 μm. However, the resulting powder contains the residual foaming agent and therefore, the purity of the powder is naturally limited. In addition, it was difficult to further micronize the resulting powder and to further increase the rate of hollow particles in the materials according to this method.
Patent Document 1: JP-B-4-37017.